1. Field of the Invention
This invention relates to a method and apparatus for wireless communications and, more particularly, to adaptive power control for CDMA communication systems using successive interference cancellation.
2. Description of the Related Art
Radio links for either a portable or vehicular unit can be characterized by time-varying multipath fading, which causes the link quality to vary with time. When the transmitter and receiver are provided with channel state information (CSI), transmission schemes can be adapted to this information, enabling more efficient use of the channel. For example, in current code-division multiple-access (CDMA) cellular systems (IS-95), transmission power is adapted to maintain received power from all mobiles at the same desired level. This conventional technique of controlling the signal strength at the base station (base station) is well known to be optimal for the matched filter receiver.
In recent years, there has been much interest in improving the capacity of direct-sequence CDMA (DS/CDMA) systems through the use of multi-user detectors. Successive interference cancellation (successive interference cancellation) is a form of multi-user detection in which the strongest signal is the first detected and cancelled, while the weakest is the last detected. Although the matched-filter receiver attains its best performance under equal received power from all users, the successive interference cancellation receiver favors unequal received signal powers. Studies have been conducted for finding the received signal strengths that give, after successive cancellation, equal bit-error rate (BER) performance for all users. See, for example, R.M. Buehrer, “Equal BER performance in linear successive interference cancellation for CDMA systems, ” IEEE Trans. On Communications, Vol. 49, pp. 1250-1258 (July 2001) and G. Mazzini, “Equal BER with successive interference cancellation DS-CDMA systems on AWGN and Rician channels, ” Proc. IEEE PIMRC, pp. 727-731 (September 1995). It has been shown that, by ignoring error propagation, a geometric distribution of received powers will provide an equal BER performance for all users. See, for example, A.J. Viterbi, “Very low rate convolutional codes for maximum theoretical performance of spread-spectrum multiple access channels, ” IEEE J. on Selected Areas in Communications, Vol. 8, pp. 641-649 (May 1990). A geometric distribution of received powers that minimizes the average BER, averaged over all active users, has also been considered. See, for example, J. H. Kim et al., “Combined power control and successive interference cancellation in DS/CDMA communications, ” Proc. IEEE PIMRC, pp. 931-935 (October 2002).
In spite of the years of research in the area, power adaptation techniques in CDMA systems using successive interference cancellation have predominantly been limited in their approaches to using geometric distributions of received powers to achieve a desired BER performance result.